The present invention relates to the guidance of an aircraft so as to allow it, once inserted into the traffic of an air corridor, to retain a minimum distance of separation with the aircraft preceding it so as to prevent any risk of collision.
The problem of insertion into the traffic of an air corridor, know as xe2x80x9cfollow trafficxe2x80x9d, arose right from the beginnings of aviation, in respect of access to runways and has long been solved solely by the technique of fly by sight which consists, in respect of the pilot of an aircraft, in carrying out visual surveillance of his vicinity in order to tag the other aircraft flying nearby and especially any aircraft which may be preceding him on the same trajectory, and to act on his throttle sticks so as to preserve an apparent distance relative to this aircraft which is sufficient to avoid any risk of collision. This technique, still widely carried out, in the context of highly regulated procedures, is safe only in the event of good visibility and light traffic. Should the air traffic be somewhat heavy such as over Europe or the North American continent, the technique of fly by sight is used alongside the routing of the aircraft from the ground, by air controllers who assign positions and speeds to the various aircraft allowing them to insert themselves into the traffic of an air corridor while complying with the minimum separation distances between aircraft eliminating the risks of collision. However, even strengthened by traffic regulation by an air control authority, the safety of the fly by sight technique depends on the visibility conditions so that, in order to maintain one and the same safety level, the poorer the visibility conditions, the larger must be the minimum spacing between aircraft.
Navigation and radiocommunication equipment having progressed in terms of reliability and accuracy, it is envisaged that the extent of the domain of action of air controllers be limited to zones of heavy traffic with prescribed air corridors and that, outside of these zones, that is to say in the higher space, aircraft be granted mastery of their navigation so as to ensure better traffic flow. The downside of this autonomy of navigation in higher space is the need for the aircraft to ensure their anticollision safety by themselves. With this aim, and that of better safety, it is proposed to equip aircraft with a so-called xe2x80x9cADS-Bxe2x80x9d system, the initials standing for xe2x80x9cAutomatic Dependent Surveillance Broadcastxe2x80x9d, via which an aircraft informs the various other aircraft flying nearby and the terrestrial air control stations within its range, of its position and its speed vector, either periodically or when interrogated.
An aircraft flying amid traffic consisting of other aircraft equipped with the ADS-B system is able to ascertain its situation relative to the various other aircraft and to forecast, in the short and medium term, the way in which this situation will alter and hence the collision risks incurred in the short and medium term, if no correction is made to the trajectories of the various relevant aircraft. Armed with this knowledge, it is capable of modifying its trajectory or of requesting another aircraft to modify its trajectory so as to eliminate risks of collision. Various methods directly usable by airborne navigation computers onboard aircraft have already been proposed for detecting the risks of collisions with other aircraft and for determining the course corrections allowing these risks to be eliminated at the cost of a minimum detour.
It is proposed here that the knowledge which an aircraft possesses, ever more often, of its position and its speed vector as well as of the positions and speed vectors of the various other aircraft flying nearby, by virtue of the ADS-B system and also by virtue of other systems such as airborne radars or means for communicating with ground tagging stations, be used to automate the maintaining of a longitudinal safety distance relative to another aircraft preceding it along its scheduled course. This additional aid afforded to the crew of the aircraft and to any air control authority involved is able to reduce the spacing between aircraft for one and the same safety level and hence to increase the traffic in an air corridor.
The subject of the invention is a process for guiding a so-called slave aircraft in the context of a convoy flight, behind another so-called master aircraft, on one and the same predetermined trajectory and with a preset minimum separation distance DSEP, consisting in giving the calculated air speed VCAS,S of the slave aircraft, that is to say calculated from measurements of temperature and static and dynamic pressures and known as xe2x80x9cCalculated Air Speedxe2x80x9d, an initial value VCAS0,S, in periodically deducing the distance separating the slave aircraft from the master aircraft from the geographical position of the slave aircraft and from that of the master aircraft at one and the same instant, and from the known shape of the trajectory followed by the master and slave aircraft, then in periodically correcting the calculated air speed VCAS,S of the slave aircraft by modifying it with a corrective term E derived from the discrepancy noted between the measured separation distance D and the preset minimum separation distance DSEP, this guidance process being defined by a recurrence relation of the form:   "AutoRightMatch"      {                                                                                                                                  V                                              CAS                        ,                        S                                                              ⁡                                          (                                              t                        0                                            )                                                        =                                                            V                                              CAS0                        ,                        S                                                              ⁡                                          (                                              t                        0                                            )                                                                                                                                                                                      V                                              CAS                        ,                        S                                                              ⁡                                          (                                                                        t                          0                                                +                                                  kΔ                          ⁢                                                      xe2x80x83                                                    ⁢                          t                                                                    )                                                        =                                                                                    V                                                  CAS                          ,                          S                                                                    ⁡                                              (                                                                              t                            0                                                    +                                                                                    (                                                              k                                -                                1                                                            )                                                        ⁢                            Δt                                                                          )                                                              +                                                                                                                        E            ⁡                          (                                                (                                                            D                      SEP                                        -                                          D                      M                                                        )                                ,                                  (                                                            t                      0                                        +                                                                  (                                                  k                          -                          1                                                )                                            ⁢                      Δ                      ⁢                                              xe2x80x83                                            ⁢                      t                                                        )                                            )                                          
xcex94t being the periodicity of the correction.
Advantageously, the initial value of the calculated air speed of the slave aircraft is taken equal to the calculated air speed scheduled during the preparation of the flight plan of the slave aircraft 2 at the place where it is situated at the moment of implementation of the guidance process, this calculated air speed resulting from the application of a 4D defining law, that is to say dependent on the four variables: altitude, time, latitude and longitude, this case being tailored to a situation where the aircraft is descending, for example, on the approach to a runway.
Advantageously, the initial value of the calculated air speed of the slave aircraft is taken equal to a calculated air speed and adopted in order to optimize consumption. This is applied to the case where the slave aircraft is cruising.
Advantageously, the initial value of the calculated air speed of the slave aircraft is taken equal to the calculated air speed of the master aircraft 1 at the moment at which it passed substantially the same point of the trajectory.
Advantageously, one is given a so-called effective margin of tolerance xcex94Le, as a percent, in the measured distance DM relative to the preset distance DSEP below which the corrective term E takes a zero value and above which the corrective term takes a non zero value.
Advantageously, a hysteresis phenomenon is introduced around the effective margin of tolerance xcex94Le, by giving the latter a reduced value upon each detection of its overshoot, doing so as long as the overshoot lasts.
Advantageously, in the case where there is a preset margin of tolerance xcex94Lc, an effective tolerance margin xcex94Le which is stricter than the preset tolerance margin xcex94Lc is adopted by applying a positive safety coefficient kxe2x80x2 of less than 1 to the preset tolerance margin xcex94Lc, for example 0.8 so that the preset tolerance margin xcex94Lc is never reached:
xcex94Le=kxe2x80x2xcex94Lc with 0 less than kxe2x80x2xe2x89xa61 
Advantageously, in the case where the effective tolerance margin xcex94Le is deduced from a preset tolerance margin xcex94Lc by applying a safety coefficient kxe2x80x2, a hysteresis phenomenon is introduced around the effective tolerance margin xcex94Le by giving the latter, when it has just been overshot, a reduced value deduced from the preset tolerance margin xcex94Lc by applying a positive hysteresis coefficient kxe2x80x3 of less than the safety coefficient kxe2x80x2, for example, taken equal to 0.5; doing so for as long as the overshoot lasts.
Advantageously, the corrective term E is taken proportional to the discrepancy noted between the measured separation distance D and the preset minimum separation distance DSEP with a coefficient of proportionality p taken equal to:
p=Sepxc3x97M 
with:   "AutoRightMatch"      {                                                      S              ep                        =                          1              ⁢                              xe2x80x83                            ⁢              if              ⁢                              xe2x80x83                            ⁢                              {                                                                                                                                                                                        "LeftBracketingBar"                                                                                                (                                                                                                            D                                      M                                                                        -                                                                          D                                      SEP                                                                                                        )                                                                "RightBracketingBar"                                                                                                                    D                              SEP                                                                                 greater than                                                                                     k                              xe2x80x2                                                        xc3x97                            Δ                            ⁢                                                          xe2x80x83                                                        ⁢                                                          L                              c                                                        ⁢                                                          xe2x80x83                                                        ⁢                            and                            ⁢                                                          xe2x80x83                                                        ⁢                                                          S                              ep                                                                                                      =                                                  0                          ⁢                                                      xe2x80x83                                                    ⁢                          a                          ⁢                                                      xe2x80x83                                                    ⁢                          priori                                                                                                                                                or                                                                                                                                                                                                              "LeftBracketingBar"                                                                                                (                                                                                                            D                                      M                                                                        -                                                                          D                                      SEP                                                                                                        )                                                                "RightBracketingBar"                                                                                                                    D                              SEP                                                                                 greater than                                                                                     k                              xe2x80x3                                                        xc3x97                            Δ                            ⁢                                                          xe2x80x83                                                        ⁢                                                          L                              c                                                        ⁢                                                          xe2x80x83                                                        ⁢                            and                            ⁢                                                          xe2x80x83                                                        ⁢                                                          S                              ep                                                                                                      =                                                  1                          ⁢                                                      xe2x80x83                                                    ⁢                          a                          ⁢                                                      xe2x80x83                                                    ⁢                          priori                                                                                                                                                                              and                                                                S              ep                        =                          0              ⁢                              xe2x80x83                            ⁢              if              ⁢                              xe2x80x83                            ⁢                              {                                                                                                                                                                                        "LeftBracketingBar"                                                                                                (                                                                                                            D                                      M                                                                        -                                                                          D                                      SEP                                                                                                        )                                                                "RightBracketingBar"                                                                                                                    D                              SEP                                                                                 less than                                                                                     k                              xe2x80x3                                                        xc3x97                            Δ                            ⁢                                                          xe2x80x83                                                        ⁢                                                          L                              c                                                        ⁢                                                          xe2x80x83                                                        ⁢                            and                            ⁢                                                          xe2x80x83                                                        ⁢                                                          S                              ep                                                                                                      =                                                  1                          ⁢                                                      xe2x80x83                                                    ⁢                          a                          ⁢                                                      xe2x80x83                                                    ⁢                          priori                                                                                                                                                or                                                                                                                                                                                                              "LeftBracketingBar"                                                                                                (                                                                                                            D                                      M                                                                        -                                                                          D                                      SEP                                                                                                        )                                                                "RightBracketingBar"                                                                                                                    D                              SEP                                                                                 less than                                                                                     k                              xe2x80x2                                                        xc3x97                            Δ                            ⁢                                                          xe2x80x83                                                        ⁢                                                          L                              c                                                        ⁢                                                          xe2x80x83                                                        ⁢                            and                            ⁢                                                          xe2x80x83                                                        ⁢                                                          S                              ep                                                                                                      =                                                  0                          ⁢                                                      xe2x80x83                                                    ⁢                          a                          ⁢                                                      xe2x80x83                                                    ⁢                          priori                                                                                                                                                            
xcex94Lc being a preset tolerance percentage, kxe2x80x2 a positive safety coefficient less than one, kxe2x80x3 a hysteresis coefficient and M a consistency coefficient for making the distance homogeneous in relation to the speed.
Advantageously, the consistency coefficient M is taken equal to:   M  =            120      ⁢              xe2x80x83            ⁢      kts              1      ⁢              xe2x80x83            ⁢      NM      
kts being a speed in knots and NM a nautical mile.
Advantageously, the measured separation distance DM is the horizontal component of the true separation distance.
Advantageously, the measurement of the distance DM separating, at a given instant, the slave aircraft from the master aircraft on a rectilinear trajectory, is taken equal to:
DM=Rxc3x97Arc cos(cos(LatS)xc3x97cos(LatS)xc3x97cos(LongMxe2x88x92LongS)+sin(LatM)xc3x97sin(Lats)) 
LatM and LongM defining the geographical position of the master aircraft, LatS and LongS the geographical position of the slave aircraft and R being the radius of the earth.
Advantageously, the distance separating, at a given instant, the slave aircraft from the master aircraft while the slave aircraft is situated on a rectilinear part, with heading "khgr"P, of the trajectory followed by the two aircraft, and the master airplane on another rectilinear part, with heading "khgr"S, of this trajectory, which are joined together by an arc of a circle of radius r, starting at a point P1 marking the end of the rectilinear trajectory part traversed by the slave aircraft and terminating at a point P2 marking the start of the rectilinear trajectory part traversed by the master aircraft, is taken equal to the sum of the distances separating the slave aircraft from the point P1 and the point P2 from the master aircraft plus the length L of the circular arc taken equal to:
L=r("khgr"sxe2x88x92"khgr"P) 
the headings "khgr"P and "khgr"S being expressed in radians.
Advantageously, the separation preset distance DSEP is deduced from a minimum timespan preset xcex94T between the passings of the two aircraft (1, 2) through one and the same point of the trajectory by applying the relation:
DSEP=xcex94Txc3x97VTAS.S 
VTAS,S being the true speed of the slave aircraft in the air, known by the designation TAS speed stemming from the term xe2x80x9cTrue Air Speedxe2x80x9d.
Advantageously, the separation preset distance DSEP is derived from a defining table giving it as a function of altitude.
Advantageously, the separation preset distance DSEP is a distance preset given by an air controller.
Advantageously, the location of the master aircraft is transmitted in a cooperative manner by the latter to the slave aircraft by means of an ADS-B system.
Advantageously, the slave aircraft acquires the location of the master aircraft by means of an onboard radar.
Advantageously, the slave aircraft acquires the location of the master aircraft by way of a ground-based locating station.